Many of the bolded characters in the characterization above are apomorphies of subsets of streptophytes along the lineage leading to the embryophytes, not apomorphies of crown-group embryophytes per se.

All groups below are crown groups, nearly all are extant. Characters mentioned are those of the immediate common ancestor of the group, [] contains explanatory material, () features common in clade, exact status unclear.

Evolution.Divergence & Distribution. Branching in this general part of the tree - i.e. the Petrosaviales, [Dioscoreales + Pandanales], and Liliales clades, and also in Petrosaviales itself, may be somewhere around 125-120 m.y.a. (ca 111 m.y. in Bremer 2000b), and the stem groups of all other orders, including those in the commelinid group, have diverged by ca 115 m.y.a. or soon afterwards (Janssen & Bremer 2004) or ca 106 m.y.a. (Magallón et al. 2015). Much of this divergence may have taken place in Southern Gondwana, i.e. Antarctica, Australasia, and southern South America (Bremer & Janssen 2006).

For a discussion on leaf development in monocots, see the Acorales page.

Phylogeny. Relationships between commelinids, Asparagales, Dioscoreales, Liliales, and Pandanales were unclear for some time. In a smallish early study, Liliales were sometimes embedded in Asparagales (Eguiarte et al. 1994). A three-gene (rbcL, atpB, 18S RNA) study (Chase et al. 2000a) showed a polytomy of Petrosaviaceae, Dioscoreales, Pandanales, Liliales, Asparagales and commelinids, although a single shortest tree showed a pectinate structure with the taxa in the sequence followed here; another analysis with placeholders for taxa missing some sequences gave a similar structure, except that Pandanales and Liliales were sister taxa. (A combined morphological plus molecular tree in the same volume [Stevenson et al. 2000] suggested a substantially different set of relationships; bootstraps were not given.) Fay et al. (2000a) also suggested a sister relationship between Asparagales and commelinids, although sampling outside Asparagales was sketchy since it was outside their immediate interest. Hilu et al. (2003: matK) i.a. suggested that Orchidaceae might be separate from other Asparagales (the latter being sister to commelinids) and that Dioscoreales and Pandanales formed a clade.

However, a two-gene (matK, rbcL) study (Tamura et al. 2004a) began to clarify the situation. Petrosaviaceae (both genera - but see below for a possible third - were studied) were sister to a clade [[Dioscoreaceae + Pandanaceae] [Liliales [Asparagales + commelinids]]] (see Chase et al. 2000a above). Support was quite high (³85% bootstrap) for all order and family branches, although rather lower for [Asparagales + commelinids] (68%) (see also Tamura et al. 2004b, a smaller study; Lam et al. 2016). Davis et al. (2004) also found Petrosaviales to be sister to the same monocots, but with moderate to weak (>72%) support. Givnish et al. (2005: ndhF gene alone) found very much the set of relationships in the tree here, although Pandanales grouped with Liliales (low support) and Dasypogonaceae were sister to [Commelinales + Zingiberales]; a grouping [Liliales [Pandanales + Dioscoreales]] also appeared - and had moderate support - in MP, but not in ML analyses of plastid genomes in Barrett et al. (2013: sampling). Graham et al. (2006) in a study analysing considerable amounts of data also recovered relationships similar to those suggested by Tamura et al. (2004a), all sister taxon relationships in this area having 94% or more support, although that for [Liliales [commelinids + Asparagales]] was only 70% (see also Givnish et al. 2006b; Chase et al. 2006; Magallón et al. 2015). Dioscoreales and Pandanales are sister taxa in most studies (e.g. Hilu et al. 2003; Chase et al. 2006; Qiu et al. 2010: support strong; Magallón et al. 2015; Hertweck et al. 2015; Lam et al. 2015).

There is perhaps still some uncertainty. In some reconstructions Dioscoreales and Pandanales are adjacent along the spine (e.g. Janssen & Bremer 2004; Bremer & Janssen 2006; Givnish et al. 2006b: not strongly supported), or Nartheciaceae link with Pandanales, the combined group in turn joining with Liliales (Davis et al. 2004: summary of earlier literature on relationships of the two). A four-gene mitochondrial tree suggested the relationships [Asparagales [[Dioscoreales + Pandanales] [Liliales + Commelinids]]], but support was not strong (Qiu et al. 2010), while Davis et al. (2013) recovered a very weakly supported topology [Asparagales s. str. [Orchidaceae + Liliaceae]] in parsimony but not in maximum likelihood analyses (see also Barrett & Davis 2011). Ruhfel et al. (2014) found a [Liliales [Dioscoreales + Pandanales]] clade while in analyses of the nuclear PHYC gene alone Orchidaceae was not sister to other Asparagales - relationships were [Asparagales [Liliales [Orchidaceae + commelinids]]], but support was weak (Hertweck et al. 2015).

Some mycoheterotrophic taxa cause problems, and in an analysis using three chloroplast genes (not present in all the mycoheterotrophs), branch length could be extremely long, although by no means always (Lam et al. 2016). Neyland (2002) found that Thismia was sister to a well supported Burmannioideae, but with less support, but Burmanniaceae s.l. did not link with other Dioscoreales. Analysis of 26S rDNA sequences suggested that Corsiaceae were polyphyletic; Arachnitis perhaps being sister to Thismia and/or Burmannia (Neyland & Hennigan 2003; G. Petersen et al. 2006b: combined analysis). A recent analysis of plastid loci also failed to include Arachnitis in Liliales, and perhaps it was to be included in the commelinids (Kim et al. 2012). However, these relationships have not been confirmed, and these mycoheterotrophs seem to be finding stable resting places, as in a study by Lam et al. (2016) that included a very wide variety of mycoheterotrophs from the next five orders in the sequence here.

Classification. Since no firm association of Petrosaviaceae with any other order has been supported, and its phylogenetic position, as in the tree here, seems well supported, a monofamilial Petrosaviales is appropriate.

Note:Boldface denotes possible apomorphies, (....) denotes a feature common in the clade, exact status uncertain, [....] includes explanatory material. Note that the particular node to which many characters, particularly the more cryptic ones, should be assigned
is unclear. This is partly because homoplasy is very common, in addition, basic information for all too many characters is very incomplete, frequently coming from taxa well embedded in the clade of interest and so making the position of any putative apomorphy uncertain. Then there are the not-so-trivial issues of how character states are delimited and ancestral states are reconstructed (see above).

Evolution.Divergence & Distribution. This is not exactly a clade in which there has been much diversification (Hertweck et al. 2015).

Evolution.Bacterial/Fungal Associations.Japonolirion osense has about 22 phylotypes of Glomales associated with it (Paris-type mycorrhizae here); the mycoheterotrophic Petrosavia sakuraii has but a single one of these (Yamato et al. 2014).

Genes & Genomes. For evolution (loss of genes, etc.) in the chloroplast genome of the mycoheterotroph Petrosavia, see Logacheva et al. (2014; Luo et al. 2016); rather unusually, there has been extensive rearrangment of the genome.

Chemistry, Morphology, etc. The roots of Petrosavia have an unmedullated, four-radiate stele. Remizowa (2011) suggested that the position of the septal nectaries in both the ascidiate and plicate zone of the gynoecium might be unique and so a synapomorphy for this tiny but quite heterogeneous clade.

For general information, see Tamura (1998:
Nartheciaceae) and especially Cameron et al. (2003), for anatomy, see Stant (1970), for pollen, see Caddick et al. (1998), for sieve tube plastids, see Behnke (2003), for floral and inflorescence morphology, see Remizowa et al. (2006a, b) and Tobe (2008), and for the embryology of Petrosavia, see Tobe and Takahashi (2009: nice comparative table).

Phylogeny.Isidrogalvia schomburgkiana (Tofieldiaceae!) was placed sister to Petrosavia (L.-Y. Chen et al. 2013) in a study focusing on the aquatic Alismatales - a rather remarkable position that needs confirmation (but see Luo et al. 2016: contamination or misidentification).

Previous Relationships. Petrosaviaceae have often been included in other families. Dahgren et al. (1985) placed them - along with genera here placed in Nartheciaceae and Tofieldiaceae - in Melianthaceae, and while Tamura (1998) recognised a Petrosaviaceae, this also included members of Tofieldiaceae and Nartheciaceae. Petrosaviaceae s. str. (i.e. Petrosavia alone) were placed in Triuridales by Cronquist (1981) and in Triurididae by Takhtajan (1997); the latter included a monogeneric Japonoliriaceae in his Melanthiales-Liliidae.